Transport Refrigeration Current Limit Control During Pulldown
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Solution Overview
Problem
Conventional transport refrigeration systems face high current draw risks during the 'pull down' mode when cooling products from ambient temperatures above the desired storage temperature, potentially exceeding maximum current limits, especially in high ambient conditions.
Innovation Solution
A method to control the current draw by determining if the ambient temperature is high and the system is in pull down mode, reducing the maximum current limit if exceeded, and iteratively lowering it until the limit is three amps below the initial, then resetting upon specific conditions such as deactivation or ambient temperature changes.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Speed
If the refrigeration unit is designed with high refrigeration capacity to rapidly cool products from ambient temperature during pull down mode, then the cooling speed is improved, but the current draw by the compressor may exceed the maximum current limit
Solution Approach 1:
The patent applies dynamics by making the maximum current limit adjustable rather than fixed. The controller dynamically modifies the maximum current limit based on operating conditions (ambient temperature, pull down mode status, time since defrost cycle) to prevent compressor overload while allowing high capacity operation when safe. This resolves the contradiction by enabling the system to operate at high capacity when conditions permit while automatically reducing the current limit when overload risk is detected.
2Power
If the refrigeration unit operates at maximum load capacity during pull down mode, then the refrigeration capacity is improved, but the risk of exceeding current limits increases
Solution Approach 1:
The patent implements feedback control by continuously monitoring operating conditions (ambient temperature, pull down mode status, time since defrost cycle, current draw) and using this information to adjust the maximum current limit. The controller receives feedback about system state and modifies the current limit accordingly, ensuring the compressor operates within safe limits while maximizing refrigeration capacity when conditions allow. This feedback mechanism resolves the contradiction between high power operation and current limit compliance.
3Reliability
If the maximum current limit is reduced to prevent overload, then the reliability is improved, but the cooling capacity during pull down mode is reduced
Solution Approach 1:
The patent resolves this contradiction through dynamic adjustment of the maximum current limit based on real-time operating conditions. Rather than using a fixed reduced limit that would always constrain capacity, the system raises the limit when conditions are safe (low ambient temperature, not in pull down mode, sufficient time since defrost) and lowers it when overload risk exists. This dynamic approach maintains reliability while preserving maximum refrigeration capacity when conditions permit.
Data Source
AI summary
A method for controlling the operation of a transport refrigeration system to limit current drawn by a compressor powered by AC electric current includes the steps of: (a) determining whether an ambient temperature in which the refrigeration unit is operating has been greater than a set point ambient temperature for a first time period; (b) determining whether the refrigeration unit has been operating in a temperature pulldown mode; (c) determining whether the AC electric current is equal to or exceeds a preset maximum current limit; (d) determining whether a time period between a last defrost cycle and a next previous defrost cycle is less than fifteen minutes; and (e) if the determination is YES in both of step (a) and step (b) and is also YES in at least one of step (c) and step (d), reducing the preset maximum current limit to a reset maximum current limit.